The present invention relates to a method of manufacturing cutting tool inserts preferably for machining of heat resistant materials.
For machining of cast iron and heat resistant materials, ceramic inserts based on alumina are usually used. Such inserts are very sensitive to thermal cracks since the material has relatively low thermal conductivity. This leads to short tool life under conditions leading to thermal shocks, i.e., high edge temperatures, short engagements times and varying cutting depths.
To a certain extent, the thermal properties of alumina ceramic inserts can be improved by the addition of TiC and/or TiN whereby the thermal conductivity increases. But at the same time, the toughness behavior increases only moderately and the material can only be used at relatively low feeds.
A later step in the development comprises additions of fine-grained ZrO.sub.2 -particles to the alumina matrix. The transformation of the metastable ZrO.sub.2 -particles during use increases both strength and toughness. The thermal properties are only slightly better.
It has more recently been shown that the fracture toughness of a ceramic material can be increased by addition of monocrystalline hair crystals (whiskers) and/or platelets. A material consisting of SiC-whiskers in an alumina matrix is disclosed in U.S. Pat. No. 4,543,345. U.S. Pat. No. 4,867,761 discloses the use of carbides, nitrides or borides of Ti and/or Zr in an alumina matrix. Through U.S. Pat. No. 4,849,381, a cutting tool comprising a mixture of whiskers and platelets is known.
The combination of Al.sub.2 O.sub.3 -matrix+SiC-whiskers is commercially available as inserts for machining. Such inserts are generally manufactured by uniaxial pressure sintering. This method is, however, unsuitable for inserts with more complex geometries.
Inserts of more conventional materials, e.g., non-reinforced ceramics and cemented carbide are usually manufactured by tool pressing. In this method, powder is compacted to a solid body in a tool comprising a die and two punches. Granulated powder comprising the ceramic material and one or more binders, e.g., polyvinylalcohol and with good flow properties is filled into the die and compacted by pressing the upper punch with a certain force against the lower, usually fixed, punch. Through a suitable shape of the punches, the insert is given a suitable chip breaking geometry. A drawback with tool pressing is that the relative density varies within the body. This variation in relative density is particularly large for complex bodies with large differences in compacted height.
Also, material reinforced with whiskers and/or platelets can be tool pressed. Hereby, very high compaction pressures are needed in order to obtain a reasonable relative density. In order to obtain an even filling in the tool also in this case granulated powder is needed. The most common method for granulation is spray drying. However, during spray drying the whiskers are drawn into the middle of the granules and the granule borders are partially depleted of whiskers. In addition, the granules that are armoured by the whiskers and also contain a binder cannot be completely crushed during the compaction. This leads to an uneven structure in the sintered material.
Injection molding is a common production method in the plastics industry for the production of "net-shape" or "near-net-shape" parts. A paste comprising thermoplastic or thermohardening polymers is heated to a suitable temperature and forced through a nozzle into a form with desired geometry. The technique is also used in powder metallurgy for the manufacture of relatively complex parts starting from metallic or ceramic powders. Hereby, the metallic or ceramic powder is mixed with polymers. After the shaping, the polymer is driven off and after that the part is sintered in essentially the same way as for corresponding tool pressed parts.